1. Field of the Invention
The present invention relates in general to dental prostheses, and more particularly to a prosthetic assembly comprising a pillar intended to receive a crown or a bridge, and a pillar screw for joining the pillar to an implant implanted in the patient""s jawbone, the said pillar screw comprising a body that can be joined to the implant and a head arranged so as to project from the patient""s gum and form, in conjunction with the pillar, through complementarity of shapes, a single volume that is intended to receive the crown or the bridge.
2. Description of the Prior Art
In general, the pillar has a bottom end that is adapted on the one hand to the configuration of the implant to which it is joined, and on the other hand to the head of the pillar screw with which it interacts.
In fact, the implant can assume all kinds of configurations, such as an internal or external hexagon, or an internal or external octagon, or a cylinder provided with external or internal grooves. The implant can also have, in its inside wall, vertical grooves, in particular three vertical grooves arranged, when viewed in cross-section, at the vertices of a triangle, and the pillar then has, at its bottom end, corresponding vertical ribs that are intended to engage in the said grooves.
Furthermore, usually the pillar screw has a threaded body and passes through the bottom end of the pillar and screws into the body of the implant, thereby fixing the pillar on the implant.
The head of the pillar screw interacts with the bottom end of the pillar situated in the patient""s gum so that the pillar screw and the pillar form an integral assembly fixed in the implant. The said head is generally cylindrical and is located inside the internal diameter of the pillar. Between the head of the screw and the pillar there is a lateral clearance to enable the said head to slide, the external surface of the pillar being alone in relation to and cemented to the inside surface of the prosthesis.
The difficulty with a prosthetic assembly of this kind is long-term maintenance of fixing of the pillar, by means of the pillar screw, on the implant.
In fact, after a certain length of time during which the dental prosthesis is regularly subjected to the stresses of chewing for example, it may be found that the fixation of the pillar screw in the implant has deteriorated and that some clearance has appeared between the pillar screw and the implant, consequently causing an equivalent looseness of the pillar carrying the crown or bridge relative to the implant, which is unacceptable.
Various solutions have already been proposed for avoiding this problem.
Among these solutions, we may cite the use of a pillar screw made of gold, which acts on the inside walls of the implant as a lubricant, reducing the friction between the thread of the said screw and that of the implant, which has the beneficial effect of increasing the tightening torque between the screw and the implant.
However, the use of pillar screws in twenty-four carat gold is by no means economical.
Another solution consists of providing an interaction of surfaces called a Morse taper between the head of the pillar screw and the bottom end of the said pillar, forming an anti-unscrewing system.
This interaction of tapered surfaces requires a special conformation of the internal and external surfaces of the head of the pillar screw and of the pillar, which is relatively complex.
Furthermore, these solutions cannot guarantee a reliable result.
Document WO 97/14372 describes such a solution in which the screw head has two parts, with one part extending inside the pillar and forming anti-rotating means with the latter, through interaction of tapered internal surfaces having the same angle. The second part of the screw head extends above the pillar and has grooves or flats that are intended to interact with the prosthesis to prevent it turning relative to the prosthetic assembly.
However, in the solution according to this document WO 97/14372, the part of the pillar that projects from the gum is very short and represents a small part of the single volume formed through the interaction of the screw head and the pillar. Thus, this projecting part of the pillar represents a very small supporting surface for the crown or bridge, and the prominent part that will mainly interact with the crown to hold it in place on the prosthetic assembly, whether in rotation or in axial translation, is formed by the head of the screw.
The short pillars of the aforementioned type are difficult to position because, with the head of the implant being located under the gum, they do not remain stationary during tightening of the pillar screw, but tend to turn in the sense of rotation of the screw. Moreover, it is necessary to use a controlled-torque contra-angle attachment, which increases the cost, the complexity and the force exerted on the implant.
In this context, the present invention proposes a new solution, which is simple and economical, for reinforcing the mechanical retention of the pillar screw in a dental implant.
More particularly, the invention proposes a prosthetic assembly comprising a pillar that is intended to receive a crown or bridge, and a pillar screw for joining the pillar to an implant implanted in a patient""s jawbone, the said pillar screw comprising a body that can be joined to the implant and a head arranged so as to project from the patient""s gum and interact with the pillar to form, through complementarity of shapes, a single volume that is to receive the crown or bridge. It is characterized in that the pillar represents at least about half of the said single volume in such a way that the external surfaces of the pillar and of the head of the pillar screw are able to be adapted to the internal surface of the crown or bridge to form anti-rotation surfaces of the said crown or bridge.
Thus, according to the invention, advantageously, the head of the pillar screw and the pillar are joined to one another by the adhesive cement permitting fixing of the crown or bridge on the said pillar, which makes it possible to reinforce the fixing of the said pillar screw in the implant and avoid any subsequent inopportune separation.
Other advantageous, non-limiting characteristics of the prosthetic assembly according to the invention are as follows:
the initial volume formed by the head of the pillar screw interacting with the pillar is greater than the final volume for receiving the crown or bridge, it being possible for the said head and/or pillar to be milled by the practitioner to adjust their shape to the final volume desired;
the pillar has an internal structure adapted to the external shape of the head of the pillar screw so that the said head and the said pillar comprise complementary supporting zones;
the complementary supporting zones of the head of the pillar screw and of the pillar are flat surfaces or portions of conical surfaces or cylindrical surfaces;
the head of the pillar screw has at its base, roughly at the junction with its body, a supporting surface that is to bear on a complementary supporting surface of the pillar provided just above its bottom end and intended to interact with the implant, moreover with a clearance between the screw head and the pillar;
the pillar screw is made in two separate parts, the body and the head, assembled together by an anti-rotation means which is preferably achieved through interaction of shapes between the two parts;
the head of the pillar screw has a roughly conical or cylindrical shape;
the pillar screw is made of pure titanium or of titanium alloy, or of gold alloy;
the pillar is made of pure titanium or of titanium alloy or of ceramic or of sinterable material.
The description that follows, referring to the appended drawings, given as non-limiting examples, will explain the essence of the invention and how it can be carried out.